First off: Why it shouldn't work. The main plot involves people trying to take over the criminal empire of or, if that fails, kill Knucky. Since Knucky is played by Steve Buscemi, we know that their efforts are doomed to fail, so there's no dramatic tension. The secondary story lines involve people trying to kill or at least tame young versions of Al Capone, Myer Lansky and Lucky Luciano. Now any student of history knows these three people were not killed when they were still low level thugs, so there's no dramatic tension there either. We know how those story lines will pan out. The only tension possible is wondering if they are going to pull a Ned Stark on Jimmy or Chalkie - but I find either scenario highly unlikely.
Why it does work: The women. It's weird because this is a show where the men get most of the screen time, but it's the female characters that are by far the most interesting and make the overall story work. Margaret's transition from servant to abused housewife to lady of the manor and budding criminal mastermind is beautifully done. It's complex and real and you never know where they are going to go with it. Whatever happens with her siblings is going to be fascinating. Jimmy's mom is fascinating as well. The mixture of love and hate she has for the father of her child who date raped her when she was very young is complex and interesting as well. There are layers to it. Lucy could be completely one dimensional as a showgirl/hooker who got knocked up by the wrong guy. But she too takes her character into unexpected and fascinating places. Jimmy's wife is also full of surprises. She can play the role of "good wife" to Jimmy when he's around, but she's not this doting submissive thing. There's stuff deep within her that could make her the most explosive character of them all. Even the servants are interesting - and you know something going to pop up with the dark haired servant girl and the IRA bomber.
So there you have it. Boardwalk Empire - a show about men, that only works because of the women.